The present invention relates to a process for the preparation of .alpha.,.beta.-ethylenic aldehydes which contain at least 7 carbon atoms, by reaction of a compound containing a 1,3-dienoxy group with an allyl alcohol. More specifically, the present invention relates to a process for obtaining .alpha.,.beta.-unsaturated aldehydes having a terpene structure.
German application No. 1,768,552 has proposed the preparation of .alpha.,.beta.-ethylenic aldehydes containing at least 7 carbon atoms, and more particularly, of .alpha.,.beta.-ethylenic aldehydes containing at least one unit of the general formula: ##STR1## by reaction of allyl alcohols of various structures with dienyl ethers of the general formula: ##STR2## or their precursors, derived from an enolisable .alpha.,.beta.-ethylenic aldehyde or .beta.,.gamma.-ethylenic aldehyde, by heating in the presence of various catalysts. This process constitutes an elegant means of arriving, in particular, at terpene aldehydes such as citral, farnesal or .alpha.-sinensal. These aldehydes, as well as the majority of their homologues, are of great industrial interest as intermediates in organic synthesis (citral can be used as an intermediate for the synthesis of vitamin A or of perfumes such as citronellal or hydroxycitronellal) and/or as perfumes or as additives in the foodstuff industry.
In spite of its value, the process described in said German application No. 1,768,552 is not free from disadvantages. Among these, the most objectionable is the difficulty of obtaining the dienyl ethers used as starting compounds. These ethers are generally prepared by passing acetals of .alpha.,.beta.-ethylenic aldehydes or .beta.,.gamma.-ethylenic aldehydes, or .beta.-alkoxyacetals of saturated aldehydes, over catalysts at a high temperature (250.degree. to 350.degree. C.); the synthesis of these acetals is also delicate. Thus, the 1-alkoxy-3-methyl-butadienes (1-methoxy- or 1-ethoxy-isoprenes) can be obtained by passing the dimethyl-acetal or diethyl-acetal of .beta.-methylcrotonaldehyde, or 1,1,3-triethoxy- or 1,1,3-trimethoxy-3-methyl-butane over a catalyst based on acid magnesium phosphate deposited on sodium silicate. In turn, the 1,1,3-trialkoxy-methyl-butanes are obtained by reaction of an alkyl vinyl ether with acetone acetals, which can be obtained either by acetalisation of acetone or by reaction of a lower alcohol with a vinyl ether [compare I. N. NAZAROV et al., J. Gen. Chem. USSR 29, 116-123 (1959); I. N. NAZAROV et al. Doklad, Akad, Nauk. U.S.S.R., 117, 823-5 (1957)].
In addition to the disadvantages associated with the accessibility of the dienyl ether starting materials, the implementation of the process described in the above-mentioned German application necessitates the use of catalysts which favor the condensation reaction, such as inorganic or organic proton acids, or their salts, or Lewis acids. Furthermore, it appears that this process is only applicable with difficulty in a case where the dienyl ether does not contain a hydrocarbon substituent (for example, a methyl radical) on the carbon atom located in the .beta.-position relative to the ether group; in that case, an .alpha.,.beta.-ethylenic aldehyde, resulting from the attachment of the allyl radical to the carbon in the .beta.-position relative to the ether group, is mainly formed [cf. A. F. Thomas, J. Am. Chem. Soc. 91, 3281-3289 (1969)]. According to this author, the presence of a hydrogen atom on the carbon atom in the .beta.-position relative to the ether group would block the reaction at an intermediate stage. The ultimate effect is that the process described in said German application No. 1,768,552 lends itself poorly to the preparation of .alpha.,.beta.-ethylenic aldehydes which contain a methyl group in the .beta.-position relative to the aldehyde group but are unsubstituted in the .alpha.-position, such as citral and farnesal.
It is, therefore, an object of the present invention to provide a process for preparing .alpha.,.beta.-ethylenic aldehydes containing at least 7 carbon atoms without the requirement of a catalyst.
It is also an object of the present invention to provide a process for preparing .alpha.,.beta.-ethylenic aldehydes containing at least 7 carbon atoms by condensation of readily available compounds having a system of conjugated double bonds with an allyl alcohol.
Other objects will be apparent to those skilled in the art from the present description.